(a) Field of the Invention
The present invention relates to a method for preparing molded articles of syndiotactic polypropylene and the molded articles. More specifically, it relates to a method for molding syndiotactic polypropylene which comprises the steps of melting, molding and stretching syndiotactic propylene homopolymer having a relatively high tacticity, syndiotactic copolymer of propylene and a small amount of ethylene or another .alpha.-olefin having a relatively high tacticity, or a mixture of the same and isotactic polypropylene, and it also relates to the thus obtained molded article.
(a) Description of the Prior Art
Syndiotactic polypropylene has been known for a long period of time. However, syndiotactic polypropylene prepared by polymerization at a low temperature in the presence of a conventional catalyst comprising a vanadium compound, an ether and an organic aluminum compound was poor in syndiotacticity and had elastomer-like characteristics. Therefore, the thus prepared polypropylene was hardly considered to have characteristic properties as syndiotactic propylene. In recent years, polypropylene having good tacticity, i.e., a syndiotactic pentad fraction of more than 0.7 has been discovered for the first time by J. A. Ewen et al. which can be obtained by the use of a catalyst comprising a transition metal compound having asymmetric ligands and an aluminoxane (J. Am. Chem. Soc., Vol. 110, p. 6255-6256, 1988).
In a molded article which can be prepared by molding, in a usual molding manner, syndiotactic polypropylene obtained by the above-mentioned method, a copolymer, having a substantially syndiotactic structure, of propylene and ethylene or another .alpha.-olefin, or a mixture of the same and a small amount of polypropylene having a substantially isotactic structure, impact resistance is relatively excellent, but stiffness is not so sufficient. In consequence, this kind of molded article is not considered to be excellent in the balance of physical properties.